This invention relates to an improved method for the manufacture of a pour-in-place (PIP) composite article. These composite articles comprise a porous exterior covering and a flexible polyurethane foam core. The improvement in the pour-in-place method according to the present invention comprises applying pressurized gas to the outside surface of the exterior covering at the point directly opposite the position that the reaction mixture will be injected into the mold either at some point prior to the reaction mixture is injected into the mold, or simultaneously at the point the reaction mixture is injected into the mold, and discontinuing the application of pressurized gas after the injection step is completed.
Most furniture and automotive vehicle seats, armrests and headrests have traditionally been produced from molded polyurethane foam cushions or parts. Subsequent to the foam molding process, the part is wrapped in pre-cut and pre-sewn fabric covers. This "pre-cut and sew" method has drawbacks in that it is very labor intensive and the seats produced by this method tend to deteriorate rapidly or pockets form after repeated use. It is also difficult to achieve a consistently perfect shape from seat to seat and to produce concave contours.
To overcome deficiencies of the "pre-cut and sew" method, the so called "pour-in-place" method was developed in the 1970's. The pour-in-place method involves pouring polyurethane foam reactants in a liquid form onto a pre-shaped cover and then allowing the foam to expand and cure to form an in-situ foamed molded part. Conventional pour-in-place (PIP) technology is known and described in, for example, U.S. Pat. Nos. 4,860,415, 4,959,184, 4,976,414, 5,124,368 and 5,360,831. In the 1970's, the pour-in-place method was applied for the production of office furniture and other simple-shaped articles such as tractor seating using impermeable PVC sheet covers. The pour-in-place method using fabric covers was introduced subsequently and is now referred to as the foam-in-fabric or foam-in-cover method.
In pour-in-place foaming technology, the exterior cover material can be a fabric backed by a thin layer of foam and a urethane film barrier (the so called "barrier" technique), or a fabric backed only by a thin layer of foam. Using the barrier technique, production of molded polyurethane foam filled articles by the PIP process is very efficient.
These "barrier" techniques may include textile composites with or without foam interliners laminated with a nonpermeable film backing. These are placed in a mold and shaped to fit the contours of the mold by applying vacuum. See 33.sup.rd Annual Polyurethane Technical/Marketing Conference, Sep. 30-Oct. 3, 1990, D. Murphy et al, Pp. 172-176 and F. W. Schneider et al, Pp. 32-39; as well as U.S. Pat. No. 4,738,809 and EP-A-1 901 828 and EP-A-1 181 604. The non-permeable film serves two purposes: (a) when vacuum is applied between the mold surface and the laminate, no air is sucked through the laminate and the laminate is pressed upon the mold surface and (b) the foam formulation which is poured in a liquid state on top of the laminate cannot penetrate or strike-through the non-permeable layer. This eliminates unacceptable stiffening and staining of the fabric. However, since the laminate is non-permeable to any fluid or gas, there is a definite drop in thermal comfort of the seats produced with this technique. This is a clear disadvantage for car seats because lack of "breathability" makes car seats particularly uncomfortable to use for an extended period of time.
When exterior covers are only foam backed (i.e., no urethane film is present), the liquid foaming reaction mixture frequently penetrates the foam layer and forms a hard spot on the fabric surface. Although this fabric type is much less expensive to use due to the absence of the urethane film barrier, this hard spot defect results in rejects during production, a loss in efficiency and costly manual recovery of the armature or support. The cover is not reusable and the cost of cover material and the manual sewing step is also lost.
Methods of increasing reaction speed and foam viscosity, well known in the industry, are ineffective at resolving this production problem. Improvement can be made by lowering machine output but this increases cycle time and lowers production rate. Thus, manufacturers are generally unwilling to do this.
One method of avoiding this problem, is to glue an extra piece of foam backed fabric cover material on to the interior of the presewn cover directly under where the liquid foaming urethane mixture is introduced into the cover under high pressure. This additional step, however, adds to the cost of the cover assembly.
Another solution has been to use a funnel designed to divert the reaction mixture being injected away from the bottom surface of the exterior cover where the penetration occurs. This also adds to the cost of production as these funnels are expensive to produce and more difficult to clean and reuse in production.
As discussed above, the lack of "breathability" is a clear disadvantage for car seats because this results in car seats that are particularly uncomfortable to use for an extended period of time. To overcome the "breathability" problem, a so-called "non-barrier" foam-in-fabric technique has recently been developed. According to the non-barrier technique, the fabric is laminated with a polyurethane slabstock foam layer of about 2 to 5 mm thickness but without the non-permeable fill. The slabstock foam layer can be of two types, having virtually zero or low air breathability. Low breathability slabstock foams usually have less than 1.0 cfm breathability as measured in accordance with ASTM Method D-3574 p. 9 Ref.: Air Flow Test.
When a slabstock foam layer is used which has virtually zero air breathability, it protects the fabric against liquid polyurethane foam reactants to almost the same extent that is achieved by using the non-permeable film. However, the low porosity of the foam layer produces little improvement to thermal comfort of a car seat compared to the one produced by the barrier technique.
On the other hand, when a slabstock foam backing is used which has some air breathability, it is obvious that polyurethane reactants can not be poured in liquid form since they will penetrate the foam layer creating a hard spot and staining the fabric.
In order to avoid penetration of the foam layer, two techniques are currently being used. One requires the use of foam formulations which have a very fast cream time (cream foams) as described in FR-A-2,470,566. Such cream foams, however, create a problem as foam flow is reduced and, for larger molds, filling problems occur. Thus, a cream foam technique is mainly applicable for production of small articles such as car head rests. Additionally, other problems are experienced such as fogging or chemical staining of the fabric by amine vapors due to the very high levels of amine catalysts required to produce the cream foam. The high catalyst level also leads to high foam compression set values, especially after humid ageing.
The other technique uses a pre-expansion chamber. In this technique, the liquid foam reactants, after mixing, are kept for a short time in a chamber where the reactions start. The reactive blend can, therefore, reach a creamy stage before being poured onto the fabric. A version of this device is described in U.S. Pat. No. 4,925,508. The disadvantage of this technique is that it is mechanically difficult to build a reusable preexpansion chamber due to the plugging of movable parts by the reactants.
That is the reason why, in U.S. Pat. No. 4,925,508, the pouring nozzle, acting as a pre-expansion chamber, is made out of plastic (polyethylene or polystyrene) and disposed of after each pour. The use of a pre-expansion chamber described in U.S. Pat. No. 4,925,508 will particularly be inconvenient and uneconomical for use of a typical molding line where several different parts would be produced.
It is known to use gases such as gaseous carbon dioxide as a blowing agent for molded polyurethane foam. See, for example, EP-A-0 089 796, EP-A-0 267 490, DE-OS-3,916,873, U.S. Pat. Nos. 3,821,130, 3,862,879, 4,483,894, 4,906,672 and 4,965,029. None of these references or any other known references, however, describe or even suggests the use of a non-reactive gas in a process for making low density pour-in-place or foam-in-fabric articles for cars or furniture.
In U.S. Pat. No. 5,360,831, it was disclosed that the use of an inert gas in a flexible foam formulation, and particularly a high resilience foam formulation, to froth the foam formulation when pouring the same onto a foam backing of the fabric, eliminated penetration of the foam layer even with foam layer with air flows higher than 1.0 cfm, and did not reduce the flow of the foaming mass. This was unexpected and different from the cream-foam or the pre-expansion chamber techniques described above.
The process of U.S. Pat. No. 5,360,831 relates to the preparation of pour-in-place articles, specifically car or furniture seats, arm-rests and head-rests. It comprises pouring a flexible or a semi-flexible polyurethane (cold or hot cured, particularly high resilience) foam formulation onto a pre-shaped composite cover and allowing the foam formulation to rise and cure. This process requires a sufficient amount of an inert gas to be dissolved in or dispersed in the foam formulation such that the liquid foam reactants leave the mixing head in a partially frothed state.
Gas assisted injection molding is known and described in, for example, U.S. Pat. Nos. 5,344,596 and 5,716,560. These processes comprise the injection of a flowable thermoplastic material followed by injection of at least one portion of an inert gas into the cavity. The injected gas is dispersed along the inner face of the thermoplastic material and forces it against the inner face of the mold, thereby improving the appearance or surface quality of the appearance side of the resultant molded part.
In the method for forming an armrest disclosed in U.S. Pat. No. 5,611,977, a base foaming urethane material is injected as a liquid into a preformed three-dimensional trim cover assembly within a foaming die, and allowed to foam to form the armrest body for an automotive seat. Impregnation of the foam layer with the liquid foaming material requires that the trim cover assembly comprises a permeable top cover layer, a low-density foam layer, an impregantion preventive film having plural ventiliation holes formed therein, and a high density foam layer in the order listed.
It has now unexpectedly been discovered that the penetration of the exterior cover material by the liquid foaming material into the non-barrier protected foam backing may be substantially reduced and/or eliminated by simply injecting pressurized gas onto the exterior of the fabric surface directly opposite from where the liquid stream impinges the foam backed cover. The pressurized gas stream is applied to the outside of the exterior cover material either from just before the liquid foaming material is injected from the high pressure metering equipment through the injection port, or simultaneously with the start of the injection of the liquid foaming material from the high pressure metering equipment through the injection port. The gas pressure is stopped just after the injection of the liquid foaming urethane mixture is complete. This simple step significantly reduces, if not eliminates, the formation of hard spot defects without the added cost of other solutions.
All of the known prior art processes continue to suffer problems with the foaming reaction mixture penetrating through the exterior covering of non-barrier protected cover composites. It has been found that the application of pressurized gas to the outer surface of the exterior covering directly across from the injection port through which the reaction mixture enters the inside cavity formed by the exterior covering material assists in preventing the reaction material from impregnating the exterior covering material.